RS-422 Interface Definition

RS-422 Interface Definition
RS-422 Interface Definition

What is the RS-422 interface.

The electrical properties of RS-422 are exactly the same as RS-485. The main difference is:

RS-422 has 4 signal lines: two sends (Y, Z), two receives (A, B). Due to the RS-422 of the collection and the hair

Is separate so you can receive and send at the same time (Full-duplex). RS-485 has 2 signal lines: both A and B are sent and received. Since the RS-485 and hair is a total of two lines, so can not receive and send (Half-duplex). Can the y-a of RS-422 be used as a RS-485, and the z-b of RS-422 be shortened as B of RS-485. Answer: not necessarily. The condition is that RS-422 must be able to support multiple machine communications. All of the interface converters of RS-422 can support Full-duplex multiple-machine communication, so it can be converted to RS-485.

RS422 Interface Definition

The RS-422 standard full name is "The electric characteristic of the balanced voltage digital interface circuit", it defines the characteristic of the interface circuit. Figure 9 is a typical RS-422 four-line interface. There is actually a signal ground, a total of 5 lines.



Because the receiver uses high input impedance and send drive driver is stronger than RS232, it allows to connect multiple receive nodes on the same transmission line, up to 10 nodes. That is, a master device (master), the rest from the device (salve), from the device can not communicate, so RS-422 support point-to-point two-way communication.

Figure 8 is its DB9 connector pin definition.









The input impedance of the receiver is 4k, so the maximum load capacity of the transmitter is 10x4k+100ω (terminal resistor).



RS-422 four-line interface because of the use of separate send and receive channels, so do not have to control the direction of the data, each device must be any signal exchange between the software (Xon/xoff handshake) or hardware (a pair of separate twisted pair) to achieve.



The maximum transmission distance of the RS-422 is 4000 ft (about 1219 m) and the maximum transmission rate is 10mb/s. The length of the balance twisted pair is inversely proportional to the transmission rate, and the maximum transmission distance may be reached under the 100kb/s rate. The highest rate of transmission can be achieved only at a very short distance. The maximum transmission rate on a 100-metre-long twisted pair is only 1mb/s.



RS-422 need a final resistor, the resistance is about equal to the characteristic impedance of the transmission cable. In the moment distance transmission can not be the end of the resistor, that is generally under 300 meters do not need the final resistor. The terminal resistor is connected to the farthest end of the transmission cable.